Calcium, not smoking may limit birth-control bone loss

NEW YORK (Reuters Health) - Injectable birth control is known to lower bone density, but women may be able to limit the loss by not smoking and getting even moderate amounts of calcium, a new study hints.

The findings, say researchers, show that not all women are at equal risk of bone loss from using depot medroxyprogesterone (DMPA) -- better known by the brand-name Depo Provera.

DMPA is given by injection about once every three months, and is generally considered an effective, convenient and low-cost form of birth control. The contraceptive can, however, lead to significant bone loss.

While research has shown that this lost bone mass is often regained after women stop using DMPA, there are still concerns about whether substantial bone loss is completely reversible. So limiting the decline in the first place would be ideal.

In the new study, researchers found that among 95 women who used DMPA for two years, those who smoked or had a low calcium intake were at particular risk of significant bone density loss -- defined as a decline of at least 5 percent in the spine or hip.

Current smokers were nearly four times more likely to lose that much bone mass as non-smokers were. On the other hand, the risk declined by 19 percent for every 100 milligrams (mg) of calcium a woman got each day.

Drs. Mahbubur Rahman and Abbey B. Berenson, of the University of Texas Medical Branch in Galveston, report the findings in the journal Obstetrics and Gynecology.

The study included 95 black, white and Hispanic women who were 24 years old, on average, at the outset. Their bone density was measured when they began using DMPA and two years later.

Overall, 47 percent of the women showed at least a 5 percent decline in bone density in the spine or hip.

Of those women, 44 percent were current smokers, versus 32 percent of women who lost less bone mass. The average calcium intake in the former group was 484 mg per day -- less than half of the recommended 1,000 mg for women their age.

According to Rahman and Berenson, the findings suggest that not smoking can go a long way toward limiting the bone loss associated with DMPA. The same appears true of even moderate calcium intake; women who got more than 600 mg of calcium per day had lesser bone loss -- about 2 percent or less over two years.

In other findings, women who had ever had a child were also at lower risk of significant bone loss. They were half as likely as childless women to see their bone density decline by 5 percent or more.

The results suggest that for DMPA users who have had children, do not smoke and get at least 600 mg of calcium day, "concerns about bone health are minimal," write Rahman and Berenson.

But when women do smoke or get little calcium, they add, doctors should offer them help with smoking cessation and counsel them on eating calcium-rich foods and taking supplements if needed.